U.S. patent number 5,510,676 [Application Number 08/415,781] was granted by the patent office on 1996-04-23 for high-pressure sodium lamp.
This patent grant is currently assigned to U.S. Philips Corporation. Invention is credited to Eduardus J. E. Cottaar, Peter J. Vrugt.
United States Patent |
5,510,676 |
Cottaar , et al. |
April 23, 1996 |
High-pressure sodium lamp
Abstract
A high pressure sodium lamp includes discharge vessel (20) which
is enclosed with intervening space (1) by an outer bulb (10), which
space contains a gas-fill with at least 70 mol. % nitrogen gas.
Electrodes (30a, 30b) are positioned in the discharge vessel (20)
and are each connected to a current supply conductor (40a, 40b)
which issues from the discharge vessel (20) at a respective end
(21a, 21b) thereof. The discharge vessel (20) has between the
electrodes (30a, 30b) a central portion (22) with length L and
volume V. The discharge vessel (20) contains an amalgam containing
a weight m.sub.Hg of mercury and a weight m.sub.Na r. of sodium,
whereby the following relations are fulfilled: ##EQU1## in which
m.sub.Hg and m.sub.Na are expressed in mg, and L and V are
expressed in cm and cm.sup.3, respectively. This allows for a
stable operation despite temperature fluctuations in the
environment, while breakdown in the outer bulb (10) and damage
caused by corrosion of current supply connectors (40a, 40b) is
prevented.
Inventors: |
Cottaar; Eduardus J. E. (Santa
Rosa, CA), Vrugt; Peter J. (Turnhout, BE) |
Assignee: |
U.S. Philips Corporation (New
York, NY)
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Family
ID: |
8210480 |
Appl.
No.: |
08/415,781 |
Filed: |
April 3, 1995 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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7216 |
Jan 22, 1993 |
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Foreign Application Priority Data
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Mar 16, 1992 [EP] |
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92200739 |
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Current U.S.
Class: |
313/639; 313/25;
313/637 |
Current CPC
Class: |
H01J
61/22 (20130101); H01J 61/825 (20130101) |
Current International
Class: |
H01J
61/12 (20060101); H01J 61/00 (20060101); H01J
61/82 (20060101); H01J 61/22 (20060101); H01J
061/20 () |
Field of
Search: |
;313/639,25,637 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
Neues aus der Technik, No. 4, 15 Dec. 1989. .
"The High Pressure Sodiumlamp", 1st impression, 1986, de Groot and
van Vliet, pp. 165-169. .
"White HPS lamps with a Color Temperature of 2700K", S. Carleto et
al., J. of the IES, Winter 1991..
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Primary Examiner: O'Shea; Sandra L.
Assistant Examiner: Esserman; Matthew J.
Attorney, Agent or Firm: Wieghaus; Brian J.
Parent Case Text
This is a continuation of application Ser. No. 08/007,216, filed on
Jan. 22, 1993.
Claims
We claim:
1. A saturated high-pressure sodium lamp comprising a ceramic
discharge vessel which is enclosed with intervening space by an
outer bulb, which space is provided with a gas filling, a pair of
electrodes arranged in the discharge vessel, a respective current
supply conductor connected to each electrode and issuing from the
discharge vessel at a respective end, between which electrodes a
central portion of the discharge vessel extends with a volume V and
a length L, the discharge vessel being provided with a filling of
an amalgam with a quantity m.sub.Na of sodium by weight and a
quantity m.sub.Hg of mercury by weight, characterized in that;
the gas in the space in the outer bulb comprises at least 70 mol.%
nitrogen, and in that m.sub.Hg, V and L satisfy the relation
##EQU9## and in that the weight ratio between sodium and mercury in
the amalgam satisfies the relation that ##EQU10## in which m.sub.g
and m.sub.Na are expressed in mg L in cm and V in cm.sup.3, and in
that the mercury and sodium are present in a sufficient quantity
such that mercury and sodium remain present in the liquid phase
during lamp operation throughout lamp life.
2. A high-pressure sodium lamp as claimed in claim 1, characterized
in that the weight ratio of sodium to mercury in the amalgam
satisfies the relation that ##EQU11##
3. A saturated high pressure sodium discharge lamp exhibiting
unsaturated characteristics, said lamp comprising:
a) an outer envelope sealed in a gas-tight manner;
b) a gas filling within said outer envelope including at least 70
mol.% nitrogen;
c) a discharge device within said outer envelope, said discharge
device comprising a ceramic discharge vessel, said discharge vessel
including a pair of discharge electrodes between which a discharge
is maintained during lamp operation, a pair of current supply
conductors each connected to a respective discharge electrode and
issuing from the discharge vessel in a gas-tight manner, said
discharge vessel including a central portion between said
electrodes with a volume V and a length L, and a filling of an
amalgam with a quantity of m.sub.NA of sodium by weight and a
quantity m.sub.Hg of mercury by weight,
the quantity of mercury and sodium each being selected such that
mercury and sodium each remain present in the liquid phase during
lamp operation throughout lamp life,
m.sub.Hg, V and L satisfy the relation ##EQU12## and the ratio
between sodium and mercury in the amalgam satisfies the relation
##EQU13## in which m.sub.Hg and m.sub.NA are expressed in mg, L in
cm and V in cm.sup.-3, whereby said saturated high pressure sodium
lamp is highly insensitive to external temperature changes.
4. A saturated high pressure sodium discharge lamp according to
claim 3, wherein said current conductors consist essentially of
niobium, and said discharge vessel has a temperature during lamp
operation at the location where said current conductors extend
through said discharge vessel such that, except for a thin nitride
layer, corrosion of said current conductors at their portions
exposed to said nitrogen-containing gas within said outer envelope
substantially does not occur.
Description
BACKGROUND OF THE INVENTION
The invention relates to a high-pressure sodium lamp provided with
a ceramic discharge vessel which is enclosed with intervening space
by an outer bulb, which space is provided with a gas filling, a
pair of electrodes being arranged in the discharge vessel each
connected to a current supply conductor issuing from the discharge
vessel at a respective end, between which electrodes a central
portion of the discharge vessel extends with a volume V and a
length L, the discharge vessel being provided with a filling of an
amalgam with a quantity m.sub.Na of sodium by weight and a quantity
m.sub.Hg of mercury by weight.
Such a lamp is known from Neues aus der Technik, No. 4, 15 December
1989. By "ceramic discharge vessel" is meant a discharge vessel of
a refractory material such as monocrystalline metal oxide, for
example sapphire, polycrystalline metal oxide, for example
translucent gastight aluminium oxide (PCA) or yttrium oxide, and
crystalline non-oxidic material such as aluminum nitride. The gas
filling in the space between the discharge vessel and the outer
bulb contributes to the heat transport from the discharge vessel to
the surroundings. This has the advantage that a small discharge
vessel can be used, so that the light radiated by the lamp can be
very well concentrated into a beam and a high system efficiency can
be achieved. A gas filling, however, increases the influence of the
ambient temperature on the temperature of the discharge vessel, and
thus leads to greater variations in the arc voltage of the lamp in
the case of temperature fluctuations in the surroundings. It is
stated for the unsaturated lamp, in which the sodium and the
mercury are fully evaporated during evaporation, that the vapor
pressure and thus the arc voltage and other lamp characteristics
remain substantially constant in the case of temperature
fluctuations. The use of a gas filling is for this reason
recommended for an unsaturated lamp in the above publication.
For outdoor use, it is required for the unsaturated lamp that the
amalgam be fully evaporated during operation at comparatively low
ambient temperatures. Because of the small quantity of amalgam, the
unsaturated lamp requires high temperatures at the ends of the
discharge vessel which to realize vapor pressures which correspond
to those of a comparable saturated lamp. These high temperatures in
unsaturated lamps with PCA discharge vessels have an additional
advantage because they achieve a comparatively high aluminum vapor
pressure. The comparatively high aluminum vapor pressure
decelerates parasitic chemical reactions between wall material and
sodium, by which it is prevented that the quantity of sodium, which
is already small, is lost in this way. Metal lamp components such
as lead-through elements, soldered connections between the
electrodes and the lead-through elements, and portions of current
supply conductors adjoining the ends of the discharge vessel are
also strongly heated at the prevailing high temperatures. Although
nitrogen is regarded as an inert gas in the cited publication, it
was found that this gas attacks the said metal lamp components
under the prevailing conditions in the known lamp, which leads on
the one hand to brittleness and on the other hand to a volume
increase of the attacked component. The volume increase of
lead-through elements involves the risk of the discharge vessel
cracking and starting to leak. The embrittlement of a soldered
joint will reduce the strength thereof and may even lead to a
connection being completely broken. In the case of an electrode
fastened by means of a soldered joint, this leads to tilting of the
electrode, which means the end of lamp life. A disadvantage of
argon and other rare gases is that breakdown occurs therein under
certain circumstances when an ignition voltage for starting the
lamp is applied, which renders ignition difficult and adversely
affects lamp life.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a lamp of the kind
mentioned in the opening paragraph in which at least breakdown as
well as attack of metal lamp components is avoided and whose
sensitivity to ambient temperature fluctuations is nevertheless
small.
According to the invention, the lamp of the kind described in the
opening paragraph is for this purpose characterized in that the gas
in the space in the outer bulb comprises at least 70 mol. %
nitrogen, and in that m.sub.Hg, V and L satisfy the relation and in
that it is true for the weight ratio between sodium and mercury in
the amalgam that ##EQU2## in which m.sub.Hg and m.sub.Na are
expressed in mg, L in cm, and V in cm.sup.3.
The said quantity of nitrogen prevents breakdown irrespective of
any rare gases present in the outer bulb.
As a result of the chosen quantities of ingredients, the lamp has a
saturated filling of which a quantity of mercury remains in the
liquid phase during operation, which quantity is at most of an
order of magnitude comparable to that of the evaporated quantity of
mercury, while only a small quantity of sodium evaporates. Owing to
the filling chosen in this way, it is found that a stable lamp
operation is possible which is highly insensitive to external
temperature changes. The insensitivity of the lamp operation is
based on the effect of "mercury exhaustion" which occurs during
operation and which is described in "The High Pressure Sodium lamp,
1st impression, 1986, de Groot and van Vliet, pp. 165-169". This
description, which incidentally provides no indications that this
effect can be used for practical purposes and in which also no link
is laid with the use of a gas filling in the space enclosed by the
outer bulb, gives as a guideline that this effect occurs at an
amalgam dose of between 3 and 50 .mu.g/mm.sup.3. The inventor has
found that, if this effect is to be utilized in practice, the
required mercury dose not only depends on the volume of the central
portion of the discharge vessel, but also on the length thereof.
With a mercury dose below the value resulting from the lower limit
of 0.8 for the ratio m.sub.Hg /.sqroot.(V.times.L), the mercury
vapor pressure during lamp operation is so low that the lamp
carries an inadmissibly high current under normal conditions. If
the mercury dose is in excess of the value resulting from the upper
limit of 1.7 for the said ratio, the quantity of mercury remaining
in the amalgam is too great in proportion to the quantity present
in the vapor phase, which has the result that the mercury
exhaustion required for a stable lamp operation does not occur. The
sodium dose required is dependent on the mercury dose. A weight
ratio of sodium to mercury below 0.05 or above 0.25 leads to a bad
color rendering and a low luminous efficacy.
The presence of an excess filling is essential for the mercury
exhaustion effect to occur. The temperatures prevailing at the
ends, accordingly, are low in comparison with an unsaturated lamp.
It was indeed found that a nitride layer is formed on some portions
of the current supply conductors in the lamp according to the
invention, but that a further corrosive attack does not take place
after that, so that damage is avoided.
The quantity of sodium in the discharge vessel is much greater than
in a comparable unsaturated lamp. This is not only because the
total quantity of amalgam is greater, but also because of the
greater weight ratio of sodium to mercury in the amalgam. Special
measures for limiting the loss of sodium, such as are desirable in
an unsaturated lamp, are there/ore unnecessary. The comparatively
low temperatures in the lamp according to the invention as a result
have no adverse effects, also when a discharge vessel of DCA is
used.
In spite of the saturated filling of the lamp according to the
invention, it has an advantage which it shares with the unsaturated
lamp, i.e. that the lamp does not extinguish and ignite
periodically at the end of its life.
It is noted that a lamp is known from "White HPS lamps with a Color
Temperature of 2700 K, S. Carleton et at., J. of the IES, Winter
1991", in which the outer bulb is filled with nitrogen. Lamps of
this kind, however, have a comparatively low luminous efficacy and
are accordingly uneconomical for many applications. It is noted in
the said publication that the use of a gas filling renders the
lamps more sensitive to changes in ambient temperature.
Furthermore, DE-33 07 197 A1 discloses a high-pressure sodium lamp
in which the discharge vessel is provided with a filling of which
the quantity of mercury evaporated during operation is of the same
order of magnitude as the quantity remaining in the liquid state,
while only a small quantity of sodium evaporates. The lamp,
however, has an evacuated outer bulb.
In a favorable embodiment, it is true for the weight ratio of
sodium to mercury in the amalgam that: ##EQU3## In this embodiment,
the distance in the CIE chromaticity diagram between the color
coordinates of the light radiated by the lamp and the Planckian
locus is at most approximately 0.01.
BRIEF DESCRIPTION OF THE DRAWINGS
An embodiment of the lamp according to the invention will be
explained in more detail with reference to the drawing, in
which
FIG. 1 is an elevation of a high-pressure sodium lamp according to
the invention, and
FIG. 2 shows the discharge vessel of the lamp of FIG. 1 in
longitudinal section.
It is noted that the Figures are not drawn true to scale.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 shows a high-pressure sodium lamp provided with an elongate
discharge vessel 20 with ends 21a, 21b. The discharge vessel 20 is
circular-cylindrical and has an internal diameter of 0.40 cm.
Alternatively, for example, the discharge vessel 20 may narrow
towards the ends 21a, 21b. The discharge vessel 20 made of DCA is
enclosed with intervening space 1 by an outer bulb 10 which
supports a lamp cap 50. The lamp consumes a power of 150 W during
nominal operation. The enclosed space 1 is provided with nitrogen.
The filling pressure of the gas in the enclosed space preferably
lies between 25 and 100 kPa. The pressure is then high enough on
the one hand for considerably improving the heat transport from the
discharge vessel to the surroundings, while on the other hand the
operating pressure is not so high that special measures, such as a
thick wall of the outer bulb, are required for avoiding the risk of
explosion. In the present case the filling pressure is 100 kPa.
FIG. 2 shows the discharge vessel 20 in which a pair of electrodes
30a, 30b is arranged, each electrode being fixed with titanium
solder 41a, 41b to an end 42a, 42b of a lead-through element in the
form of a niobium tube 43a, 43b which serves as a current supply
conductor 40a, 40b and which issues to the exterior at an end 21a,
21b of the discharge vessel 20. Alternatively, for example, the
lead-through element may be a rod. A central portion 22 of the
discharge vessel 20 with a length L of 4.2 cm extends up to the
electrodes 30a, 30b. The central portion 22 of the discharge vessel
20 accordingly has a volume V of 0.53 cm.sup.3. The discharge
vessel 20 is provided with a filling of an amalgam with 0.18 mg
sodium and 1.42 mg mercury. The relation ##EQU4## satisfies the
requirement that this ratio must lie between 0.8 and 1.7. More in
particular, the ratio lies between 0.9 and 1.4. It is true for the
weight ratio of sodium to mercury in the amalgam that: ##EQU5##
This ratio, therefore, lies between 0.05 and 0.25, and more in
particular between 0.10 and 0.20. 0.8 mg mercury evaporates during
operation, which is of the same order of magnitude as the 0.62 mg
remaining in the liquid state, while only a small quantity, 0.011
mg, of the sodium present evaporates.
Owing to the said composition, the lamp is highly insensitive to
ambient temperature fluctuations.
After an endurance test of 8000 hours, the current supply
conductors 40a, 40b were found to be free from corrosion.
The light radiated by the lamp has a color temperature of
approximately 2040 K. The color coordinates (x, y) of the light are
0.525 and 0.417, respectively. The distance to the nearest point
(0.524, 0.414) on the Planckjan locus is approximately 0.003, so
smaller than 0.01.
The behavior of the lamp was compared with that of a lamp whose
discharge vessel was provided with an overdose filling of 25 mg
amalgam with a weight ratio of 0.23. The lamp voltages of the lamps
were measured during operation while the lamp received/breed
cooling from a fan, during operation in neutral surroundings, and
during operation with a glass envelope present around the lamp. The
lamp voltage measured during this in V is given in the table below.
The deviation in percents from the lamp voltage during operation in
neutral surroundings is given between parentheses.
______________________________________ lamp according lamp with
overdose fill to the invention
______________________________________ with forced cooling 92
(-10%) 101 (-7%) neutral surroundings 102 (0%) 109 (0%) with
envelope 120 (+18%) 115 (+6%)
______________________________________
In a further embodiment, the lamp consumes a power of 1000 W during
operation. The gas filling in the space 1 in the outer bulb 10 of
this embodiment preferably has a filling pressure below 90 kPa.
Special measures for avoiding the risk of explosion are unnecessary
then, also when a comparatively large outer bulb is used. In a
practical implementation, the enclosed space has a filling of
nitrogen with a filling pressure of 80 kPa. The central portion 22
of the discharge vessel 20 has a length L of 10.8 cm and an
internal diameter of 1.09 cm. The volume V of the central portion
22 accordingly is 10.1 cm.sup.3. The discharge vessel 20 is
provided with a filling comprising 10.64 mg mercury and 1.86 mg
sodium. It follows from this that: ##EQU6## This satisfies the
requirement that the ratio must lie between 0.8 and 1.7. In
addition, the ratio lies between the preferred limits of 0.9 and
1.4. The weight ratio of sodium to mercury lies between 0.05 and
0.25, more in particular between 0.10 and 0.20, and is in this case
approximately 0.17. During operation 7.0 mg mercury and 0.27 mg
sodium evaporate, so that the order of magnitude of the 3.64 mg
mercury remaining in the liquid state is comparable to the order of
magnitude of the evaporated quantity. Only a small portion of the
sodium present has evaporated.
The lamp was subjected to an endurance test of 12,000 hours. A
nitride layer had formed on the current supply conductors 40a, 40b
after the endurance test, but no damage had occurred.
Furthermore, unsaturated lamps were manufactured with
nitrogen-filled outer bulbs 10. The filling pressure was 65 kPa.
The lamps were provided with discharge vessels 20 with an internal
diameter of 0.685 cm. The central portion 22 in this case has a
length L of 6.6 cm and a volume V of 2.43 cm.sup.3. The electrodes
30a, 30b were fixed to the ends 42a, 42b of niobium tubes 43a, 43b
with titanium solder joints 41a, 41b, respectively. The discharge
vessel 20 was provided with 2.7 mg mercury and 0.034 mg sodium. It
is true for the mercury dose that: ##EQU7## The sodium/mercury
weight ratio in the amalgam is: ##EQU8## The lamps consumed a power
of 400 W during nominal operation. Both the mercury and the sodium
were fully evaporated during this. In contrast to the lamps
according to the invention, the electrodes 30a, 30b in several of
these lamps had tilted against the discharge vessel 20 within 3000
hours as a result of a corrosive attack on the titanium solder 41a,
41b by nitrogen, while cracks had appeared in the discharge vessel
20.
* * * * *